This invention relates to digital imagesetting and, more particularly, to visual sensors for detecting banding.
Printing presses use plates to print ink onto paper and other media. One method used for creating plates is to expose photosensitive film with the matter to be printed. When the film is developed, the matter imaged on the film may be imaged onto a photosensitive plate, sometimes referred to as xe2x80x9cburningxe2x80x9d a plate. After processing, the plate can be used on a press to print the matter onto a medium. Part of the plate, usually the part defining the image to be printed, retains ink, while the other part of the plate does not. When the plate is introduced to ink and then to paper or other medium, the image is printed on the medium.
In a black and white printing job, there is usually one plate that is used to print black ink. In a color printing job, a different plate may be used for each color ink. A color job may use three colors of ink, usually cyan, magenta, and yellow, which in combination can be used to make other colors. A plate is usually produced for each color ink. Often, in addition to cyan, magenta, and yellow, black ink is also used. An additional plate is then required to print the black ink. Occasionally, one or more colors will be printed separately as well, referred to as a xe2x80x9cspot color.xe2x80x9d That color will also have its own plate.
Electronic prepress systems have used an imagesetter to receive raster data associated with a plate and to image the raster data onto photosensitive film. In this context, a raster may specify an image by pixels in columns and rows, at a predefined resolution. The film is then used to create a plate. The imagesetter exposes the photosensitive film pixel by pixel. One way that imagesetters image the raster data is to scan a laser across and down a piece of film. Electronics control the laser to expose, or refrain from exposing, each pixel in the raster data. The imagesetter images the pixels on the film in a manner that is precise and repeatable. Recently, platesetters also have been used to create plates directly from raster data without the use of intermediate film. Imagesetters, platesetters and like print engines, including proofers, are also referred to generally as output devices and writing engines.
Modern output devices may write or record images on various media used in image reproduction, including but not limited to photo or thermal sensitive paper or polymer films, photo or thermal sensitive coatings, erasable imaging materials or ink receptive media mounted onto an image recording surface, polymer film or aluminum based printing plate materials. Such media are mounted onto a recording surface which may be planar or curved.
Conventional digital imagesetters include a raster image processor (RIP) which receives signals representing an image to be recorded on the applicable media and converts the signals into instructions to a scanner which scans the recording media to produce the desired image. It is the function of the RIP to process the received signals representing the image into a corresponding instruction set that can be understood by the scanner.
Noise in various components of a writing engine, such as vibrations or inaccurate laser scan mechanisms for example, can result in imaging artifacts, sometimes referred to as banding.
Generally, a large component of banding artifacts are line spacing errors, meaning errors in the uniformity of line spacing, or, in other words,local variations in the address size compared to mean address over a long length of copy. High end recording systems are extremely sensitive to such noise effects. Uniformity of line spacing is often required to be less than +/xe2x88x92⅓ to +/xe2x88x92{fraction (1/10)} pixel error at maximum engine addressability. Often a specification for a high end recording system specification requires that imaging artifacts due to banding be essentially invisible for a range of possible output image patterns. This objective is hard to achieve, and it is equally hard to measure since any visible banding exceeds the threshold of failure. It is thus difficult to certify that a writing engine meets its specification, since it is difficult to determine if the writing engine meets the banding criteria.
The traditional method of qualifying a writing engine for banding is to image a full test page with a halftone test pattern that represents the maximum level of difficulty. For an engine with a maximum addressability of 3600 dpi, for example, a page might be imaged at a 96% tint level at 318 lines per inch. To test banding effects resulting from line spacing uniformity errors in the range of {fraction (1/10)} pixel requires a very dark tint, typically greater than 80% to 90%. Also, it typically requires some single pixel structures in the tint pattern. For these reasons a 96% tint is often used. Even at this high tint value, the low visibility of residual {fraction (1/10)} to ⅕ pixel line spacing uniformity errors requires a large sample size, often greater than 6xc3x976 inches to reliably detect the level of resulting banding error. For this reason a full page often is imaged for evaluation. The imaged media is compared with a separate master pass/fail reference page. Observation conditions, such as lighting, viewing angle, and condition of the master can influence the comparison. The visual appearance of the test page is also sensitive to setup/process conditions of the engine and the media. For example, the visual level of banding is generally very sensitive to exposure. A test can be made to change from pass to fail simply by increasing the exposure setting slightly. This is a critical problem when attempting to provide a consistent standard for banding pass/fail criteria. Generally, the exposure must be within 0.3D density units of correct setting, and the writing engine must be focused so that it is within 5% of its design value for every test to obtain useful banding detection results.
In addition to being difficult, the banding qualification process is time consuming and uses a large amount of mediaxe2x80x94often a full page for each test. It would therefore be useful to have a more efficient and accurate mechanism to qualify the banding effects of a writing engine.
A visual indicator is described that can be used to qualify the banding of an writing engine. The visual sensor can be imaged on a media to magnify the effect of banding to make the measurement of banding more apparent to a user or machine vision system.
In general, in one aspect, the invention relates to a visual indicator including a reference portion including an image independent of banding and a banding test portion adjacent to the reference portion including an image sensitive to banding. In one embodiment, the banding test portion includes two patterns which when superimposed together magnify the visibility of banding errors. In another embodiment, the banding test portion includes two patterns which when superimposed by first imaging a first pattern and second imaging a second image on top of the first image together magnify the visibility of banding errors. In another embodiment, the banding test portion includes two patterns which when superimposed together magnify the visibility of adjacent line spacing errors.
In another embodiment, the banding test portion includes two overlaid images imaged at different addressabilities. In another embodiment, the banding test portion includes a first image imaged at a first addressability n, and a second image superimposed on the first image imaged at a second addressability m, where mxe2x89xa7n. In another embodiment, the banding test portion includes a first image imaged at a first addressability n, and a second image superimposed on the first image imaged at a second addressability m, where m is approximately 2n. In another embodiment, the banding test portion includes a first imaged horizontal one-on/one-off lines imaged at a first addressability n, and horizontal two-on/three-off horizontal lines imaged at a second addressability m, where m is approximately 2n. In another embodiment, the banding test portion includes superimposed horizontal 1-on, 1-off lines imaged at 1800 dpi and horizontal 2-on, 3-off horizontal lines imaged at 3600 dpi.
In one embodiment, the reference portion includes a coarse pattern. In another embodiment, the reference portion includes a coarse pattern modulated in the vertical axis to simulate banding.
In another embodiment, indicator also includes at least one process check portion adjacent to one of the banding test portion and the reference portion. In another embodiment, the process check portion includes a first process check portion having a first imaging characteristic and a second process check portion proximate to the first process check portion having a second imaging characteristic, wherein the imaging characteristic of one of the first and second process check portions is less sensitive to an imaging parameter than the imaging characteristic of the other of the first and second process check portions, and wherein the imaging characteristic of the first process check portion and the second process check portion appear substantially similar for at least one imaging parameter setting range and appear different otherwise. In another embodiment, one of the first and second process check portions includes a coarse tint and the other of the first and second process check portions includes a fine tint. In another embodiment, the coarse tint is a (nxc3x97n) periodic pattern and the fine tint is a (mxc3x97m) periodic pattern such that (n greater than m). In another embodiment, the imaging parameter is at least one parameter chosen from the set of exposure setting, pulse width modulation, focus, balance, spot size, spot shape, spot ellipticity, sidelobes size, sidelobes shape, sidelobes intensity, modulation ON/OFF beam intensity ratio, media gamma, edge sharpness, dot gain, uniformity, ink receptivity of plate material, physical media changes, pattern dependent effects, sensitivity to position errors, and sensitivity to exposure errors.
In general, in another aspect, the invention relates to a method for generating a visual indicator. The method includes forming a reference portion comprising an image independent of banding and forming a banding test portion adjacent to the reference portion comprising an image sensitive to banding. In one embodiment, the banding portion includes two patterns which when superimposed magnify the visibility of banding errors. In another embodiment, the banding portion includes two patterns which when superimposed magnify the visibility of adjacent line spacing errors. In another embodiment, the step of forming the banding test portion includes imaging a first pattern and imaging a second pattern overlaid on the first pattern. In another embodiment, the step of forming the banding test portion includes imaging a first pattern at a first imaging parameter setting and imaging a second pattern overlaid on the first pattern at a second imaging parameter setting.
In one embodiment, the step of forming the banding test portion includes imaging a first pattern at a first addressability and imaging a second pattern overlaid on the first pattern at a second addressability. In another embodiment, the step of forming the banding test portion includes imaging a first pattern at a first addressability n and imaging a second pattern overlaid on the first pattern at a second addressability m, where mxe2x89xa7n. In another embodiment, the step of forming the banding test portion includes imaging a first pattern at a first addressability n and imaging a second pattern overlaid on the first pattern at a second addressability m, where m is approximately 2n. In another embodiment, the step of forming the banding test portion includes imaging a first pattern of one-on/one-off horizontal lines at a first addressability n and imaging a second pattern of two-on/three-off horizontal lines overlaid on the first pattern at a second addressability m, where m is approximately 2n. In another embodiment, the step of forming the banding test portion includes superimposing horizontal 1-on, 1-off lines imaged at 1800 dpi and horizontal 2-on, 3-off horizontal lines imaged at 3600 dpi. In another embodiment, the step of forming the reference portion includes forming a reference portion comprising a coarse pattern. In another embodiment, the step of forming the reference portion includes forming a reference portion comprising a coarse pattern modulated in the vertical (cross scan) axis to simulate banding.
In one embodiment, the method also includes the step of forming a process check portion adjacent to the banding test portion or the reference portion. In one embodiment, the step of forming the process check portion includes forming a first process check portion having a first imaging characteristic and forming a second process check portion proximate to the first process check portion having a second imaging characteristic wherein the imaging characteristic of one of the first and second process check portions is less sensitive to an imaging parameter than the imaging characteristic of the other of the first and second process check portions, such that the imaging characteristic of the first process check portion and the second process check portion appear substantially similar for at least one imaging parameter setting range and appear different otherwise.
In one embodiment, one of the first and second process check portions includes a coarse tint and the other of the first and second process check portions includes a fine tint. In another embodiment, the coarse tint is a (nxc3x97n) periodic pattern and the fine tint is a (mxc3x97m) periodic pattern such that (n greater than m). In another embodiment, the imaging parameter is at least one parameter chosen from the set of exposure setting, pulse width modulation, focus, balance, spot size, spot shape, spot ellipticity, sidelobes size, sidelobes shape, sidelobes intensity, modulation ON/OFF beam intensity ratio, media gamma, edge sharpness, dot gain, uniformity, ink receptivity of plate material, physical media changes, pattern dependent effects, sensitivity to position errors, and sensitivity to exposure errors.
In general, in another aspect, the invention relates to a banding indicator including a first pattern imaged at a first imaging parameter configuration value and a second pattern overlaid on the first pattern, the second pattern imaged at a second imaging parameter configuration value. The first and second pattern are configured so that the image resulting from the overlay of the first pattern and the second pattern is sensitive to banding. In one embodiment, the imaging parameter is addressability. In another embodiment the imaging parameter is at least one parameter chosen from the set of exposure setting, pulse width modulation, focus, balance, spot size, spot shape, spot ellipticity, sidelobes size, sidelobes shape, sidelobes intensity, modulation ON/OFF beam intensity ratio.
In general, in another aspect, the invention relates to a method for generating a banding indicator. The method includes imaging a first pattern at a first imaging parameter configuration value, and imaging a second pattern overlaid on the first pattern, the second pattern imaged at a second imaging parameter configuration value. The first and second pattern are configured so that the image resulting from the overlay of the first pattern and the second pattern is sensitive to banding. In one embodiment, the step of imaging the first pattern includes imaging the first pattern at a first addressability and the step of imaging the second pattern includes imaging the second pattern at a second addressability. In another embodiment, the step of imaging the first pattern includes imaging a first pattern at a first addressability n and the step of imaging the second pattern includes imaging the second pattern at a second addressability m, where mxe2x89xa7n. In another embodiment, the step of imaging the first pattern includes imaging a first pattern at a first addressability n and the step of imaging the second pattern includes imaging the second pattern at a second addressability m, where m is approximately 2n.
In general, in another aspect, the invention relates to a visual reference for characterizing banding indicated by a banding indicator. The indicated banding is referred to as the banding signature of the imaging device. The visual reference includes an image coarse enough to produce a tint signature that is relatively independent of banding, exposure setting, and focus setting. The image has the same mean tint level as the banding indicator, and the signature has approximately the same visual characteristics as a predetermined banding signature.
The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following description and from the claims.